Half-fill a cocktail shaker or a jar with a tight-fitting lid with ice. Add gin, lemon juice and syrup; shake vigorously for 30 seconds. Strain into a Champagne flute or coupe. Top with Champagne (or prosecco). Garnish with lemon twist and/or rosemary, if desired.
175 calories;0 g fat(0 g sat); 0 g fiber; 10 g carbohydrates; 0 g protein; 6 mcg folate; 0 mg cholesterol; 8 g sugars; 7 g added sugars; 2 IU vitamin A; 11 mg vitamin C; 3 mg calcium; 0 mg iron; 2 mg sodium; 35 mg potassium